EP 1 620 317 B1 discloses a thermoform packaging machine comprising a stretching device for stretching a cover film so as to position the printed cover film very exactly on top of a packaging trough. A photocell detects here, by means of a print mark on the cover film, the position of the print image relative to the forward feed and, consequently, the packaging trough. The print image is normally undersized and it is stretched in that a clamping bar clamps the flexible cover film a short time before the forward feed movement ends, the remainder of the forward feed movement then stretching the cover film. In addition, the clamping bar can be displaced in a direction opposite to the direction of feed so as to stretch the cover film until the desired dimension has been reached. In so doing, the tensioned cover film is sealed onto a trough which is implemented such that it is able to counteract the tension of the cover film so that an optically pleasing package can be obtained.
In addition, stretching units are known in which the cover film is heated by a heating plate in the area in which stretching takes place. Stretching of this area of the film is made possible by heat input. Especially films/foils of larger thicknesses, in particular hard films whose thickness exceeds 200 μm, necessitate a heat input so as to allow stretching of the film in the elastic-plastic range. During the subsequent forward feed movements of the cover film, an uncontrolled stretching behavior will occur up to a sealing station due to the non-uniform heat distribution in the cover film, the high accelerations when the forward feed movement is started and the deflections at the deflection pulleys. Especially when the cover film has to travel a comparatively long distance until it reaches the sealing station, this uncontrolled stretching behavior will lead to frequent readjustments and large tolerances as regards the positioning and the size of the print image relative to the packaging trough. The reason for this is also an uncontrolled shrinkage of the film web due to the areas of different temperatures existing in the cover film.
The stretched length of the heated film area results, after cooling down up to the sealing station, in an elongation of less than one percent of the print image. Therefore, the film supplier has to observe narrow tolerances with respect to the length and the position of the print image.